Method and apparatus for electrodeposition of group iib-via compound layers

ABSTRACT

Methods and apparatus are described for electrodeposition of Group IIB-VIA materials out of electrolytes comprising Group IIB and Group VIA species onto surfaces of workpieces. In one embodiment a method of electrodeposition is described wherein the control of the process is achieved by measuring an initial value of the electrodeposition current at the beginning of the process and adding Group VIA species into the electrolyte to keep the electrodeposition current substantially constant, such a within +/−10% of the initial value throughout the deposition period. In another embodiment an apparatus comprising multiple deposition chambers are described, each deposition chamber containing an anode and a workpiece, and wherein two thirds of the deposition chambers within the apparatus contain anodes comprising a substantially pure Group VIA element in their composition, and the rest of the deposition chambers contain anodes free from any Group VIA element in their composition.

FIELD OF THE INVENTION

The present invention relates to methods and apparatus for forming thinfilms of Group IIB-VIA compound semiconductor films, specifically CdTefilms, for radiation detector and photovoltaic applications.

BACKGROUND OF THE INVENTION

Solar cells and modules are photovoltaic (PV) devices that convertsunlight energy into electrical energy. The most common solar cellmaterial is silicon (Si). However, lower cost PV cells may be fabricatedusing thin film growth techniques that can deposit solar-cell-qualitypolycrystalline compound absorber materials on large area substratesusing low-cost methods.

Group IIB-VIA compound semiconductors comprising some of the Group IIB(Cd, Zn, Hg) and Group VIA (O, S, Se, Te, Po) materials of the periodictable are excellent absorber materials for thin film solar cellstructures. Especially CdTe has proved to be a material that can be usedin manufacturing high efficiency solar panels at a cost below $1/W. In agood quality CdTe solar cell absorber film, the Cd/Te molar ratio needsto be near unity.

FIGS. 1A and 1B show the two different structures employed in CdTe basedsolar cells. FIG. 1A is a “super-strate” structure, wherein light entersthe active layers of the device through a transparent sheet 11. Thetransparent sheet 11 serves as the support on which the active layersare deposited. In fabricating the “super-strate” structure 10, atransparent conductive layer (TCL) 12 is first deposited on thetransparent sheet 11. Then a junction partner layer 13 is deposited overthe TCL 12. A CdTe absorber film 14, which is a p-type semiconductorfilm, is next formed on the junction partner layer 13. Then an ohmiccontact layer 15 is deposited on the CdTe absorber film 14, completingthe solar cell. As shown by arrows 18 in FIG. 1, light enters thisdevice through the transparent sheet 11. In the “super-strate” structure10 of FIG. 1A, the transparent sheet 11 may be glass or a material(e.g., a high temperature polymer such as polyimide) that has highoptical transmission (such as higher than 80%) in the visible spectra ofthe sun light. The TCL 12 is usually a transparent conductive oxide(TCO) layer comprising any one of; tin-oxide, cadmium-tin-oxide,indium-tin-oxide, and zinc-oxide which are doped to increase theirconductivity. Multi layers of these TCO materials, both doped orundoped, as well as their alloys or mixtures may also be utilized in theTCL 12. The junction partner layer 13 is typically a CdS layer, but mayalternately be a compound layer such as a layer of CdZnS, ZnS, ZnSe,ZnSSe, CdZnSe, etc. The ohmic contact 15 may comprise highly conductivemetals such as Mo, Ni, Cr, Ti, Al, metal nitrides, or a dopedtransparent conductive oxide such as the TCOs mentioned above. Therectifying junction, which is the heart of this device, is located nearan interface 19 between the CdTe absorber film 14 and the junctionpartner layer 13.

FIG. 1B depicts a “sub-strate” structure, wherein the light enters thedevice through a transparent conductive layer deposited over the CdTeabsorber which is grown over a substrate. In the “sub-strate” structure17 of FIG. 1B, the ohmic contact layer 15 is first deposited on a sheetsubstrate 16, and then the CdTe absorber film 14 is formed on the ohmiccontact layer 15. This is followed by the deposition of the junctionpartner layer 13 and the transparent conductive layer (TCL) 12 over theCdTe absorber film 14. As shown by arrows 18 in FIG. 1B, light entersthis device through TCL 12. There may also be finger patterns (notshown) on the TCL 12 to lower the series resistance of the solar cell.The sheet substrate 16 does not have to be transparent in this case.Therefore, the sheet substrate 16 may comprise a sheet or foil of metal,glass or polymeric material.

The CdTe absorber film 14 of FIGS. 1A and 1B may be formed using avariety of methods. For example, U.S. Pat. No. 4,388,483 granted to B.M. Basol et al., describes the fabrication of a CdS/CdTe solar cellwherein the thin CdTe film is grown by a cathodic compoundelectrodeposition technique at low electrolyte temperatures, and thenthe as-deposited n-type CdTe film is type-converted to p-type through ahigh temperature annealing step to form the rectifying junction with anunderlying CdS layer. The compound electrodeposition or electroplatingtechnique typically uses acidic aqueous electrolytes and forms highquality rectifying junctions after the type-conversion step, yieldinghigh quality solar cells.

Present inventions provide methods and apparatus for the control ofproperties of electrodeposited Group IIB-VIA compound layers, such asCdTe thin films, in a manufacturing environment.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a cross-sectional view of a prior-art CdTe solar cell with a“super-strate” structure.

FIG. 1B is a cross-sectional view of a prior-art CdTe solar cell with a“sub-strate” structure.

FIG. 2 shows an electrodeposition system with a Group VIA materialdosing system.

FIG. 3 shows top view of an exemplary CdTe electrodeposition system withmultiple deposition chambers.

FIG. 3A shows a cross sectional side view of the system of FIG. 3 takenacross “W-W” plane.

FIG. 3B shows a cross sectional side view of the system of FIG. 3 takenacross “Y-Y” plane.

FIG. 4 shows a cross sectional side view of an electrolyte tank and adeposition chamber with two electrolyte feed lines.

DETAILED DESCRIPTION OF THE INVENTION

In general, the present invention forms high quality Group IIB-VIAcompound films, such as CdTe films at high yield in a manufacturingenvironment using an electrodeposition technique. The electrodepositionprocess is carried out of acidic solutions (also referred to as baths orelectrolytes) with a pH range of 1-3. The plating solutions orelectrolytes may comprise a high concentration of the Group IIB materialand a low concentration of the Group VIA material. For example, for CdTeelectrodeposition, an electrodeposition electrolyte may comprise >0.1M(larger than 0.1 molar) cadmium and only 0.00001-0.001M tellurium.

To keep the tellurium to cadmium molar ratio (i.e. Te/Cd ratio) in anelectrodeposited CdTe film near unity may be challenging since thecomposition of the film is a function of many factors, such as thetellurium concentration in the bath, mass transfer of the electrolyteonto the surface of the workpiece over which the CdTe film is beingplated, the deposition potential, the temperature of the electrolyte,and the quasi rest potential (QRP). QRP is the potential of the surfaceof the depositing CdTe film with respect to the plating solution underopen circuit conditions, i.e. no current flowing through the workpiece,which is the cathode. In laboratory scale work carried out of smallplating vessels, it is customary to control the composition of thedepositing CdTe film through QRP measurements (see, for example,Panicker et al., Journal of Electrochemical Society, vol. 125, page.566). This is accomplished by terminating the deposition process atcertain time intervals by cutting off the cathodic deposition current,and measuring the voltage of the CdTe film surface deposited on thecathode with respect to a reference electrode dipped in the platingelectrolyte. The deposition potential applied to the cathode is thenadjusted to keep the QRP value within a predetermined range with respectto a reference electrode such as an Ag—AgCl reference electrode orstandard calomel electrode. In a large scale manufacturing approach,many pieces of substrates need to be coated with CdTe film at the sametime, preferably using a single deposition chamber. This way cost ofmanufacturing may be kept low. Controlling all the variables listedabove, and especially measuring and controlling QRP for every singlesubstrate being coated with CdTe is not practical in such cases. Presentinventions provide methods and hardware to achieve control of thequality of electrodeposited Group IIB-VIA compound layers, such as CdTefilms, in manufacturing environments.

FIG. 2 shows an exemplary electrodeposition system 20 comprising a firstplating cell 21A, a second plating cell 21B, and a solution orelectrolyte tank 22. Group IIB-VIA compound layers may beelectrodeposited using the system 20 of FIG. 2. We will from now ondescribe various embodiments of the inventions using CdTe as an exampleof a Group IIB-VIA material. It should be noted that the methods andapparatus described may be adapted for the electrodeposition of otherGroup IIB-VIA materials including, but not limited to, zinc telluride,mercury telluride, cadmium zinc telluride, cadmium mercury telluride,zinc mercury telluride, cadmium selenide, zinc selenide, etc. It shouldbe noted that the exemplary electrodeposition system 20 has two platingcells. It is possible to add more plating cells to this system and thushave a capability to process tens of, even hundreds of, workpieces atthe same time.

Referring back to FIG. 2, CdTe films may be electrodeposited onto asurface “S1” of a first workpiece 23A, and onto a surface “S2” of asecond workpiece 23B, placed into an electrolyte 30 filling the firstplating cell 21A, and the second plating cell 21B, respectively. Itshould be noted that the first workpiece 23A or the second workpiece 23Bmay comprise a transparent sheet, a transparent conductive layer and ajunction partner layer as depicted in FIG. 1A, in which case the surface“S1” or the surface “S2” would be the exposed surface of the junctionpartner layer. Alternately, the first workpiece 23A or the secondworkpiece 23B may comprise a sheet substrate and an ohmic contact layeras depicted in FIG. 1B, in which case the surface “S1” or the surface“S2” would be the exposed surface of the ohmic contact layer.

The electrolyte 30 may be fed into the first and second plating cellsthrough a feed line 24 that connects the tank 22 with the plating cells21A and 21B. A pump 25 may pull a portion of the electrolyte 30 out ofthe tank 22 and flow it into the first plating cell 21A through a firstvalve 26A and into the second plating cell 21B through a second valve26B. One or more pumps may be used. After filling the first and thesecond plating cells, the electrolyte 30 may be returned into the tank22 as shown by arrow 28. Other means and equipment, such as heaters,filters, etc., may also be used in the system 20 of FIG. 2 to heat up,clean and filter the electrolyte 30 as it circulates between the tank 22and the plating cells. Alternately, the tank 22 may have an additionalcirculating loop (not shown) with another pump that may pump theelectrolyte 30 out of the tank 22, circulate it through filters, etc.and then return it back to the tank 22.

The two exemplary workpieces, i.e. the first workpiece 23A and thesecond workpiece 23B, may be coated with CdTe films in the first andsecond plating cells, respectively. During electroplating, using a powersupply (not shown), a negative voltage may be applied to the firstworkpiece 23A (first cathode) with respect to a first anode 27A, and asimilar voltage may be applied between the second workpiece 23B (secondcathode) and a second anode 27B. This way CdTe films may be deposited onthe surfaces “S1” and “S2” of the first and the second workpieces,respectively. In a preferred embodiment, the first workpiece 23A and thesecond workpiece 23B are electrically shorted together and connected tothe negative terminal of a single power supply. Similarly, the firstanode 27A and the second anode 27B may be electrically shorted togetherand connected to the positive terminal of the power supply. This wayonly one power supply can be used to provide voltage to the first andsecond workpieces with respect to the first and second anodes. DuringCdTe deposition, the voltage is kept constant, and the depositioncurrent flowing through each workpiece is measured and monitored. Itshould be noted that the power supply may be a potentiostat, in whichcase, a reference electrode may be dipped into the solution 30 and thevoltage of the cathode(s) may be controlled with respect to thereference electrode. It should also be noted that the electricalconnections to the anode(s) and cathode(s) are not shown in FIG. 2 tosimplify the drawing.

As discussed before, the properties of an electrodeposited CdTe layermay depend on various parameters of the electrodeposition process. Theseparameters include current, voltage, temperature, electrolyte flow, andbath composition. While investigating the interdependencies betweenthese parameters and the CdTe film quality, the present inventordetermined that best repeatable results in a manufacturing environmentcould be achieved if the deposition current and the bath composition areselected as the two variables, the deposition current being the“monitored variable” and the Group VIA material concentration of thebath being the “adjusted variable”. Accordingly, in an embodiment of thepresent inventions, the deposition current passing through at least oneof the cathodes (i.e. the first workpiece 23A and the second workpiece23B) is continually or periodically monitored during CdTeelectrodeposition, and Te species are added into the electrolyte to keepthe deposition current in a pre-determined range. For example, thedeposition current density for a good quality CdTe layer may be in arange of 0.05-0.5 mA/cm² depending on the size of the workpiece (lowercurrent densities being more appropriate for larger workpieces). Let usassume that the predetermined current density is 0.1 mA/cm² and that theallowed variation for this value is 10%. In this case, theelectrodeposition process would be initiated under constant voltage modeand the deposition current or current density would be monitored. As theCdTe film is formed over the workpiece, the Te concentration in the bathwould be depleted and the deposition current density would start to godown from the initial value of 0.1 mA/cm². Once the current densityvalue falls below the allowable value of 0.09 mA/cm², an electricalsignal may be sent by a control circuit or computer to a dosing system31 containing a Te source 32. The dosing system 31 may then dispense apredetermined amount of the Te source into the tank 22 through a nozzle33. The Te source 32 may be in the form of a liquid or solid. Apreferred form of the Te source is TeO₂ particles 32A dispersed in aliquid, preferably water, as shown in FIG. 2. Alternately, the pH of theliquid may be adjusted to be equal to the pH of the electrolyte. Astirring mechanism 32B may be used in the dosing system 31 to keep theTeO₂ particles well dispersed all the time. Alternately, the stirringmechanism 32B helps to dissolve the TeO₂ particles in case the pH of theliquid is adjusted to a low value, which may be in the range of 1-3,preferably in the range of 1-2. After the predetermined amount of the Tesource 32 is dispensed into the tank 22 and mixed with the electrolyte30, the deposition current would start to rise to the acceptable level.This process of “sensing the deposition current decline, determining ifand when the Te source addition is needed, and adding the Te source intothe electrolyte” is repeated until a predetermined thickness (such as1-2 um) of a CdTe film with uniform composition is obtained. Since thedeposition current density is kept constant at a fixed depositionpotential by controlling the Te content of the electrolyte, theresulting CdTe film has the desired composition with Cd/Te molar rationear 1.0.

In another embodiment, controlled amounts of tellurium species are addedinto the electrolyte or plating bath of a multi cell or multi chamberelectrodeposition system, from a predetermined number of anodes placedin a predetermined number of the plating cells or chambers. CdTeelectrodeposition process requires six (6) electrons, two (2) electronsfor the reduction of dissolved cadmium species in the electrolyte intoCd on the cathode surface, and four (4) electrons for the reduction ofdissolved tellurium species in the electrolyte into Te on the cathodesurface. To keep the amount of dissolved tellurium species (such asHTeO₂ ⁺ ions) in the electrolyte relatively constant during theelectrodeposition process and thus keep the deposition current valuesrelatively constant, a deposition system 40 shown in FIG. 3 may be used.The deposition system 40 of FIG. 3 is viewed from the top and itcomprises multiple chambers 42A, 42B, 42C, 42D, 42E and 42F, withinwhich CdTe may be electrodeposited on multiple workpieces. The chambersare positioned alongside an elongated tank 41 so that a plating solutionmay be circulated between the elongated tank 41 and the chambers. Thedeposition chambers of FIG. 3 may be of two different types. Forexample, the deposition chambers 42A and 42C may be “type I depositionchambers” and the deposition chambers 42B, 42D, 42E and 42F may be “typeII deposition chambers”. Type I deposition chambers have anodescomprising an inert material such as iridium oxide, titanium, platinum,etc. or elemental cadmium. Type II deposition chambers, on the otherhand, have anodes comprising tellurium. FIG. 3A and FIG. 3B show sidecross sectional views taken along planes “W-W” and “Y-Y” of the type Ichamber 42A and type II chamber 42E, respectively. As can be seen fromthese figures the plating solution 45 flows (shown by arrows 46) fromthe elongated tank 41 into the chambers 42A and 42E. The platingsolution 45 then flows back into the elongated tank 41 as shown byarrows 47. The workpieces 47A and 47B are placed into the type Ideposition chamber 42A and the type II deposition chamber 42E,respectively, for processing. The type I deposition chamber 42A containsa type I anode 48A, and the type II deposition chamber 42E contains atype II anode 48B. It should be noted that all type I depositionchambers (in this example; 42A and 42C) contain type I anodes and alltype II deposition chambers (in this example; 42B, 42D, 42E and 42F)contain type II anodes. Type I anodes may comprise an inert materialthat does not dissolve into the electrolyte 45 during processing.Alternately, type I anodes may comprise cadmium which would dissolveinto the electrolyte 45 during processing. Type II anodes, on the otherhand, may comprise substantially pure Te so that Te species dissolveinto the plating solution 45 during processing.

The number of type II deposition chambers in deposition systems of thepresent invention is double the number of type I deposition chambers. Inthe exemplary deposition system 40 of FIG. 3, the number of type IIdeposition chambers (42B, 42D, 42E and 42F) is four and the number oftype I deposition chambers (42A and 42C) is two. By selecting this kindof configuration and keeping the CdTe deposition current substantiallythe same in all deposition chambers, the concentration of the telluriumspecies in the electrolyte may be kept relatively constant in acontinuous operation. For example, in the deposition system 40 of FIG.3, as CdTe is electroplated on the workpieces in the depositionchambers, type II anodes within the type II deposition chambers 42B,42D, 42E and 42F, each would contribute to the electrolyte, throughanodic dissolution, a concentration of Te species that is proportionalto 6N, where N is the number of the type II deposition chambers, and 6is the total number of electrons needed for CdTe formation. The type Ianodes, would not contribute any Te species to the electrolyte duringthe process since they do not contain any Te. The consumption of Te inthe system 40, on the other hand would be proportional 4M, where M isthe total number of deposition chambers including the type I and type IIdeposition chambers, and 4 is the number of electrons needed at thecathode to reduce dissolved tellurium species to Te. As can be seen, inthe exemplary system 40 of FIG. 3, 6N=6×4=24, and 4M=4×6=24. Therefore,all the Te produced by the type II anodes is consumed on the cathodesfor CdTe deposition and there is no need to monitor the Te concentrationof the plating solution or to have an external dosing system to add Tespecies into the electrolyte.

As specified before, in one aspect of the present invention the numberof type II deposition chambers in a CdTe electrodeposition system isnearly double the number of type I deposition chambers. For example, adeposition system may have one type I and two type II depositionchambers, or fifty type I and one hundred type II deposition chambers,or one hundred and twenty type I and two hundred and forty type IIdeposition chambers, depending on the volume of manufacturing desired.Although FIG. 3 shows an example where the deposition chambers are alongone side of the elongated tank, other designs comprising depositionchambers distributed all around the tank in various configurations arealso possible. Since type II anodes introduce tellurium species into theelectrolyte and the type I anodes do not, there may be a differencebetween the concentration of tellurium species within the type I and thetype II deposition chambers, the electrolyte within the type IIdeposition chambers comprising a higher concentration of telluriumspecies. To avoid this problem, the flow rate of the electrolyte fromthe tank into the deposition chambers and back to the tank needs to becarefully selected. If the flow rate is very low, then the higherconcentration of tellurium species in the type II deposition chamberswould produce more Te-rich CdTe films, and the deposition current wouldalso be higher at a given deposition potential. This is not acceptablein a manufacturing environment where the electrodeposited film qualityneeds to be similar for all deposition chambers. Therefore, theelectrolyte flow needs to be adjusted so that the volume of the platingsolution contained in each deposition chamber is replaced at least 10times, preferably 20 times and most preferably at least 50 times duringthe deposition period. For example, in the deposition system 40 of FIG.3, the volume of the plating solution 45 in each deposition chamber maybe 5 gallons and the total deposition time may be 5 hours. In thisexample, the flow rate of the plating solution into each depositionchamber needs to be more than about 0.16 gallons/minute, preferably morethan about 0.33 gallons/minute and more preferably more than about 0.8gallons/minute. This way, most of the tellurium species generated by thetype II anodes within the type II deposition chambers are quickly flowninto the tank 41 (see for example arrow 47 in FIGS. 3A and 3B) and theyget mixed up with the rest of the plating solution before the solutionwith the replenished tellurium species get distributed between all thedeposition chambers.

In yet another embodiment, type II deposition chambers may employseparators or dividers. Use of such separators may reduce or even removeany constraints on the electrolyte flow rate described above. FIG. 4shows an exemplary type II deposition chamber 50 next to a solution tank51. Compared to the one depicted in FIG. 3B, the type II depositionchamber of FIG. 4 has two electrolyte feed lines, a first feed line 52and a second feed line 53, that bring electrolyte 54 into twocompartments separated by a porous divider 56. For this purpose, one ormore pumps (only one shown) may be used. The first compartment 55Acontains a type II anode 57, which comprises Te. The second compartment55B contains a workpiece 58 which acts as a cathode. The porous divider56 offers a high resistance to electrolyte flow between the first andsecond compartments. Valves 59 may be present on the first feed line 52and the second feed line 53 to regulate the flow entering the firstcompartment 55A and the second compartment 55B. In this design theelectrolyte flow rates up through the first compartment 55A and upthrough the second compartment 55B may be independently controlledduring processing. Even if the tellurium species concentration increasesin the first compartment 55A due to injection from the type II anode 57,this does not affect the CdTe deposition on the workpiece 58 in thesecond compartment 55B, because the second compartment 55B alwaysreceives a fresh mixed solution through the second feed line 53. Thefresh mixed solution, as explained before is a mix of all solutionscoming from all the type I deposition chambers and all the type IIdeposition chambers and thus contains the proper concentration oftellurium species. With the design of FIG. 4 a low electrolyte flow maybe established in the second compartment 55B for CdTe electrodepositionand a high electrolyte flow may be established in the first compartment55A to provide the tellurium species to the electrolyte 54 in thesolution tank 51.

Although the present invention is described with respect to certainpreferred embodiments, modifications thereto will be apparent to thoseskilled in the art.

1. An apparatus for electrodeposition of a Group IIB-VIA compound layeronto multiple workpieces during a deposition period, from an electrolytecomprising Group VIA species, the apparatus comprising; a tank, multipledeposition chambers, each containing an anode and at least one of theworkpieces and configured to be connected to the tank so that theelectrolyte can be circulated at a predetermined electrolyte flow rateby a at least one pump between the multiple deposition chambers and thetank, wherein two thirds of the multiple deposition chambers eachcontains an anode comprising a substantially pure Group VIA element inits composition, and one third of the multiple deposition chambers eachcontains an anode without any Group VIA element in its composition. 2.The apparatus in claim 1 wherein the predetermined electrolyte flow rateis adjusted such that a volume of the electrolyte in each depositionchamber is exchanged at least 10 times during the deposition period. 3.The apparatus in claim 1 wherein the Group VIA element is Te and theGroup IIB-VIA compound layer is a CdTe layer.
 4. The apparatus in claim1 further comprising porous dividers placed between the anode and theworkpiece within the two thirds of the multiple deposition chambers toform a first compartment around the anode and a second compartmentaround the workpiece.
 5. The apparatus in claim 4 further comprising afirst feed line and a second feed line that are configured to regulatethe electrolyte flow coming from the at least one pump and entering thefirst compartment and the second compartment, respectively.
 6. Theapparatus in claim 5 wherein the first feed line and the second feedline are configured such that the electrolyte flow entering the firstcompartment is larger than the electrolyte flow entering the secondcompartment.
 7. A method of electrodepositing a Group IIB-VIA compoundlayer on a workpiece surface immersed in an electrolyte comprisingdissolved Group IIB ionic species and dissolved Group VIA ionic species,the method comprising, immersing an anode into the electrolyte, applyinga negative voltage to the workpiece surface with respect to the anode,measuring a value of a deposition current passing through the anode andthe workpiece surface, and periodically adding a source of Group VIAionic species into the electrolyte to keep the value of the depositioncurrent substantially constant.
 8. The method of claim 7 wherein theGroup IIB-VIA compound layer is CdTe and the source of Group VIA ionicspecies comprises tellurium oxide.
 9. The method of claim 8 wherein thesource is liquid comprising tellurium oxide particles.